The present invention generally relates to fabrication of semiconductor devices and more particularly to a measurement of profile of a semiconductor structure formed on a wafer during a fabrication process of the semiconductor device.
In a production line of semiconductor devices, it is necessary to examine the size of the structures such as agate pattern formed on a wafer quickly, without contacting to or destroying the structure, such that the result of the measurement is fed back immediately to the production line for optimizing various parameters of the production line. Particularly, there is a demand for an exact control for gate lengths, as a gate length provides a profound effect on the threshold characteristic of the semiconductor device that uses a gate structure.
Conventionally, a wafer carrying thereon a structure such as a gate pattern is subjected to a scanning process conducted under a scanning electron-microscope (SEM) for evaluation of the size of the structure. Further, there is a known process for evaluating the gate pattern size by measuring a resistance of the wafer by a bridge circuit that is formed commonly to the gate pattern on the same wafer.
However, the examination process that uses a SEM takes a substantial time due to the need of transporting each wafer on the production line consecutively to a vacuum chamber of the SEM. Thus, it is not practical to apply such an SEM process to all of the wafers on the line. Further, even when the SEM process is applied only to selected wafers, a decrease of throughput is inevitable for the production of the semiconductor devices. In the case of recent miniaturized semiconductor devices having a gate length, or other structural parameters, of 0.1 .mu.m or less, in particular, the foregoing SEM process tends to cause an error in the result of the measurement due to the finite or non-infinitesimal diameter of the focused electron beam used in the SEM as compared with the size of the structure, wherein the magnitude of the error can reach as much as 10 nm.
In the case of measuring the resistance by using the bridge circuit, on the other hand, the result of the measurement cannot be obtained until the fabrication of the semiconductor devices on the wafer is completed, although the problem pertinent to the case of using a SEM such as the poor accuracy or reproducibility of the measurement may be resolved successfully. Thus, the process cannot be used for an in-situ feedback control of the production line.
Meanwhile, the art of ellipsometry has been used in the fabrication of semiconductor devices for measurement of thickness of semiconductor films and insulation films. Further, the ellipsometry is used also for controlling an etching process at the time of formation of line-and-space patterns (Blayo, N., et al., "Ultraviolet-visible ellipsometry for process control during the etching of submicrometer features,", J. Opt. Soc. Am., A, vol. 12, no. 3, 1995, pp. 591-599).
FIGS. 1A and 1B show the construction of an ellipsometer using conventionally for ellipsometry, wherein FIG. 1A shows a rotary-photometry type apparatus while FIG. 1B shows an extinction-photometry type apparatus.
Referring to FIG. 1A, the ellipsometer includes an optical source 1 for emitting an optical beam, wherein the optical beam emitted from the optical source 1 is converted to a linearly polarized beam having a predetermined plane of polarization and the linearly polarized beam thus formed hits a specimen 3 on which a film to be measured is formed. After reflection by the specimen 3, the linearly polarized beam is converted to an elliptically polarized beam characterized by an angle .o slashed. indicating the direction of the major axis of the ellipse and an ellipticity k defined as k=a.sub.min /a.sub.max as indicated in FIG. 2, wherein the parameters .o slashed. and k are related to ellipsometric parameters .psi. and .DELTA. to be used later in the description according to the relationship EQU tan 2.psi.=tan 2.o slashed..multidot.cos .DELTA. and EQU sin 2.chi.=sin 2.o slashed..multidot.sin .DELTA.,
where there holds a relationship of EQU tan .psi.=.rho..sub.p /.rho..sub.s and EQU tan .chi.=k=a.sub.min /a.sub.max.
In other words, it is possible to convert the set of the parameters (k, .o slashed.) obtained by the photometry to the parameters (.psi., .DELTA.). It should be noted that the parameter .DELTA. represents the phase shift of the optical beam.
The elliptically polarized beam thus formed is then detected by a detector 5 after passing through a rotatable analyzer 4, wherein the ellipsometer of FIG. 1A carries out the detection of the intensity of the optical beam reaching the detector 5 while rotating the analyzer 4. Further, a quarter-wavelength plate 4a, which induces a phase shift of a one-quarter of wavelength in the optical beam passing therethrough, may be inserted between the analyzer 4 and the specimen 3 as necessary.
In the ellipsometer of FIG. 1B, on the other hand, a rotary quarter-wavelength plate 4b is inserted between the rotary analyzer 4 and the specimen 3, and the elliptically polarized beam reflected by the specimen 3 is converted once to a linearly polarized beam. The rotary analyzer 4 is thereby rotated in search of the extinction angle in which the optical beam reaching the detector 5 is interrupted.
As explained previously, the ellipsometer of FIGS. 1A or 1B has been used successfully for the measurement of film thickness in the fabrication process of semiconductor devices. On the other hand, it should be noted that such conventional ellipsometry has hitherto discarded the information about the lateral size of the structure, which the polarized optical beam has inherently picked up when passing through the structure laterally. In conventional ellipsometry, there has been no proposal to make use of the ellipsometry for measuring the lateral size of the structure such as a line-and-space pattern formed on the specimen based upon the polarization state of the optical beam passed through the structure laterally.